British Pat. No. 1,337,669 discloses a process for precipitation of ionic metal impurities from phosphoric acid by adding an organic water soluble solvent and alkali metal or ammonium ions thereto, filtering off the precipitate, then passing the filtered solvent/phosphoric acid phase through a strongly acid cation exchange resin and, finally, removing the solvent from the purified acid (by distillation).
U.S. Pat. No. 3,642,439 to Moore et al, discloses a process for removing magnesium impurities from weak, wet-process phosphoric acid by evaporating the acid until the SiO.sub.2 content of the acid is reduced to less than 0.2%, maintaining the evaporated acid at 50-100.degree. C. for at least 15 hours to form a precipitate comprising a magnesium-aluminum-fluoride-phosphate complex compound and separating the precipitate from the purified, concentrated phosphoric acid. Further disclosed is the addition of a "seeding" compound having a weight ratio of magnesium oxide to aluminum oxide of 1 to 1.4 and a ratio of magnesium oxide to fluorine of 1 to 2.2. In the present invention where the calcium fluoride precipitant is an impure sludge obtained from pond water by precipitation with ca.sup.+, some magnesium can be present in the sludge; however, the weight ratio MgO to Al.sub.2 O.sub.3 of the sludge is in the range of about 1 to 6 to about 75 and the weight ratio MgO to F is typically about 1 to 23 to about 260.